Platelet accumulation is a hallmark of hemostasis and a contributor to heart attacks and strokes. In previous work, we and others have focused on identifying molecules that support platelet activation. Here we attempt a paradigm shift, approaching platelet activation as the product of a flexible signaling network rather than a collection of pathways, and joining testable ideas about the operation of that network to high resolution imaging of platelet activation in vivo. Our goal is to understand how the platelet signaling network shapes the hemostatic response and how the hemostatic response impacts the network. Our initial studies show that, rather than a homogeneous mass of platelets and fibrin, the response to penetrating injuries yields a hierarchical structure comprised of distinct regions that vary in the extent of platelet activation, packing density and porosity. A prominent feature is a core of fully-activated platelets overlaid with an unstable shell of less-activated platelets, but additional domains are present as well. Greater packing in the core allows adjacent platelets to interact, generating contact-dependent signals. Within the core is a region where thrombin activation and fibrin deposition occur. Our hypothesis is that all parts of this structure are subject to the platelet signaling network, with the core providing thrombus stabilit and gradients of soluble agonists determining the extent of thrombus growth and the size of the shell. Aim #1 will test and extend this model, applying additional activation markers, developing new methods for tracking individual platelets in vivo, and exploiting a newly- developed thrombin biosensor. Aim #2 will map relationships between the platelet signaling network and the structure of the hemostatic mass, and reassess the impact of anti-platelet agents in the context of a hierarchical model of hemostasis. Aim #3 will test the novel hypothesis that contact-dependent events are separated spatially and temporally into sets that either promote or restrain the thrombus core. Through these aims, we hope to determine why multiple agonists and signals are needed to shape an optimal platelet response, account for differences in the impact of antiplatelet agents, and show how pathological conditions can subvert normal responses by effects on the platelet signaling network.